[bugfix] Fixes assignment of BCRSMatrix with nonmatching number of rows.

When assigning a BCRSMatrix to another BCRSMatrix which was setup for
a different size/pattern, it segfaulted and I got the following
Exception:

"InvalidStateException [allocate:.../dune/istl/bcrsmatrix.hh:2108]:
Rows have already been allocated, cannot allocate a second time."

The problem was, that inside the assignment operator the rows (r) were
never deallocated. This commit fixes this and adds a testcase.

Testcase slightly modified by Markus.